Field of the Invention
This invention relates to sheet stackers such as U.S. Pat. No. 3,321,202 granted to M. D. Martin on May 23, 1967 and U.S. Pat. No. 3,419,266 granted to M. D. Martin on Dec. 31, 1968. Specifically, the foregoing sheet stackers handled corrugated and paper board sheets used in making boxes.
The Martin U.S. Pat. No. 3,419,266 sheet stacker was provided with an accumulator device which caught the trailing edge and two lead corners of the sheets as they fell from the downstream end of the conveyor. Sheets having die cuts which did not form rigid corners presented problems in accumulating sheets. Long sheets with such weak corners which exceeded the extension capacity of the cross bar holding the trailing edge of the sheet and a portion of the midportion of the sheet could not be accommodated.
Sheets having die cuts which cause flaps or tabs to hang downwardly as the sheet exits the downstream end of the conveyor are sometimes bumped by the leading edge of the rapidly extending cross bar of the Martin U.S. Pat. No. 3,419,266 accumulator causing the sheet to be skewed and preventing a straight stack from being formed. Such jams in the stacker require stopping the production line which not only causes a delay but more serious problems result if a box printer or a die cutter is in the upstream production line. Such stoppages can require throwing away partially cut or printed sheets before the production line can be restarted.
In those production lines where the sheets are shingled; i.e. overlapped on the conveyor and are discharged in a continuous stream from the discharge end of the conveyor without gaps or interruptions between sheets, the problem of extending a cross bar or fingers into this stream of sheets at a selected point so as to maintain an accurate count of sheets in a particular stack is not only difficult because the leading edge of the fingers strike the sheets and skew them, but the problem is exacerbated by the flaps and tabs which hang down below the plane of the sheet member.